[Action Corner] Not subscribe to cursor position when no action is

configured for corners

It is to avoid adding any input monitor when users are not using action
corner (i.e. all actions are None)

Bug: 426536832
Test: ActionCornerSettingRepositoryTest
Test: ActionCornerRepositoryTest
Flag: com.android.systemui.shared.cursor_hot_corner
Change-Id: I3f995d40c3d5c5e5e49362fa9b7d4caea73f4e38